hum...if I used Aqua Plus which supposedly neutralizes heavy metals, would that work? :\

No, because calcium and magnesium (ions that make up GH) are not heavy metals, Heavy metals are metals in the periodic table between copper and lead.

I keep my neon tetras in water with a pH of 6.0 to 6.2, their colours are vibrant and they are very energetic, they have been there for about a year. I have glowlights in the same tank that are up to 4 years old.
My water is very soft (GH 4 KH 1-3) Sometimes I have to use Methyl Red (I know a Biologist) to make sure the pH hasn't droped below 6.
All my amazon fish are vibrant and behave very energetic.
I think breeding the fish in captivity has made them more tolerant of hard to a certain degree.

Up until recently I have not been testing my tank for GH and KH, mainly because I didn't have a test kit. Well now I do and the latest test I have done is as follows:

pH = 7.0
GH = 60ppm
KH = 40ppm

The tank has been running over a year, I have always done regular water changes, the tank is a medium density planted tank of about 260 litres.

I tested my tap water I use for water changes today and the results were almost the same except pH was 7.2.

My question is due to much reading I have done recently regarding KH and how it assists stabilisation of pH. If I am right it appears a KH of 40ppm is quite low and would be prone to pH fluctuations.

Should I try to increase my KH, to be honest until very recently I havent really been monitoring my pH too carefully (only ammonia, nitrites and nitrates) so do not know how much the pH has been fluctuating.

If your tank has been running for a year now and the pH is at 7.0 then I don't think you have much to worry about. It means that your maintenance and water change routine is taking care of replenishing enough buffers in the tank water.

Trying to raise your KH will usually result in a pH raise as well...whether that's good for you or not depends on which species you keep. If you're happy with the pH around 7.0, just keep monitoring it weekly and if it drops below 6.6 or so you can compensate and increase KH by adding a little bit of baking soda._________________Success with a fish/tank is measured in YEARS, not months or weeks...

I just lost most of my school of praecox and celebes because of what the LFS said was an extreme reaction to the difference in the hardness between their tank water and my water. Theirs tests at over 700ppm total hardness, and mine is about 50. Water in my area is really hard, and the store doesn't mess with it.

Reason for the difference is because there is a softener in my house, that reduces the hardness to almost 0 ppm. And the guy said that not much will live in my tanks. But I have fish that are two years old and have had no problems, this has been the only incident of a mass die-off.

I can only get cold hard water out of my plumbing, but I can't put that into the tanks at water change time because the temp shock would be too much. And I think that the sudden upward swing in hardness would probably upset them too.